Gases

On Monday we started talking about gases and gas laws including kinetic-molecular theory of gases, the "simple" gas laws, and finally the ideal gas law. We'll look at gases again on Wednesday and probably start reviewing for the exam next Monday.

There's a new MC, due Sunday.

I got a question in email about the Redox assignment in MC. It's a little terminology issue that can cause some confusion so let me try and handle it here. If a substance is oxidized it loses electrons. (That's the "OIL" part of "OIL RIG" or the "LEO" part of "LEO GER".) Because oxidation and reduction cannot take place independently (if you have one, then you must have the other), you could say that if substance A loses electrons and is oxidized, then it will cause something else, let's say substance B, to be reduced. So if A is oxidized, then it is acting as a reducing agent toward B. Stated simply:

If something is oxidized, then it is a reducing agent or reductant.

If something is reduced, then it is an oxidizing agent or an oxidant.

These definitions are all about the cause-and-effect relationship between oxidation and reduction.

If you have any specific questions/topics you'd like to review for the exam, let me know.

Stoichiometry practice...

Friday in class we looked at a problem set to practice stoichiometry. It's copied below. There is a new MasteringChemistry assignment posted, due Friday. Next week we will look at gas laws and continue with stoichiometry. Your next exam is October 5th.

Chem 150 – Fall 2009 – Problem Set #2

You are studying the reaction of 1.132M potassium phosphate solution with 1.275M barium nitrate solution.

1. Write a balanced chemical equation for this reaction.

a. How many grams of precipitate could you make if you completely react 125.0mL of the potassium phosphate solution?

b. How many grams of precipitate could you make if you completely react 175.0mL of the barium nitrate solution?

c. How many mL of the barium nitrate solution is required to react completely with 125.0mL of the potassium phosphate solution?

c. How many mL of the potassium phosphate solution is required to react completely with 175.0mL of the barium nitrate solution?

d. What is the theoretical yield of precipitate (in grams) if you react 125.0mL of the potassium phosphate solution with 175.0mL of the barium nitrate solution?

e. What is the limiting reagent in part d? How many moles of the excess reagent remain after the reaction is complete?

f. Write the balanced net ionic equation for this chemical process.

2. These questions deal with concentrations of the solutions used above:

a. What is the concentration of potassium ions in the 1.132M stock solution? Phosphate ions?

b. What is the concentration of barium ions in the 1.434M stock solution? Nitrate ions?

c. How many grams of potassium phosphate are present in 125.0mL of the 1.132M stock solution? How many grams of barium nitrate are present in 175.0mL of the 1.275M stock solution?

d. If you dissolved the mass of potassium phosphate in part c in enough water to make 300.0mL of solution, what would the concentration be? If you dissolved the mass of barium nitrate in part c in enough water to make 300.0mL of solution, what would the concentration be?

e. If you dilute 125.0mL of the potassium phosphate stock solution to a total volume of 300.0mL, what will be the “new” concentration of potassium phosphate? If you dilute 175.0mL of the barium nitrate stock solution to a total volume of 300.0mL, what will be the “new” concentration of barium nitrate?

Things that go boom...

Today we looked at a couple more reactions including redox. The key to redox reactions is being able to figure out oxidation numbers for substances from their chemical formulas and it takes practice.

Practice? Yes indeed! There's a new Mastering Chemistry assignment, due Tuesday.

Introducing...Stoichiometry!!

Today we looked at the relationship between mols of reactants and products, stoichiometry. For any stoichiometry problems, there are a logical set of steps to follow:

1. Balance the equation.

2. Convert the quantity you know into mols.

3. Convert mols of what you know to "mols of interest" using the coefficients (ratios) in the balanced equation.

4. Convert "mols of interest" into a useful number. That useful number might be a mass or a volume or a number of other things, it all depends upon the problem.

We looked at a precipitation reaction and an acid-base reaction today, we'll look at a couple other reactions on Wednesday.

There is a new MasteringChemistry assignment posted, due Sept. 28th.

More on recognizing reaction types

Friday we talked about a couple classes of reactions and how to recognize them. We looked at precipitation reactions which required use to develop solubility rules to identify ionic compounds which we wouldn't expect to be soluble in water. Next, we looked at some molecule-forming reactions, starting with acid-base reactions, so we talked about acids and bases which react to form a salt and water (a molecule). We also looked at some gas-forming reactions, those that form a gas molecule when they react. We will continue looking at classes of reactions Monday, and there will be a new MC assignment on Monday.

Balancing equations...

Today I handed back exams, the average was around 125/150. For those of you who didn't do as well as you might have liked or missed the exam, remember that your low exam score is dropped in determining your final score for the course, so whether it's an exam that you miss or just the lowest score out of your 4 exams, it will not impact your course score directly.

We started talking about balancing reactions today. Your textbook introduces balancing reactions in Chapter 3 and the really gets into it in Chapter 4. We introduced things today and will really get into it on Friday. See you then. (Or tomorrow if you're in one of my labs...)

Speaking of labs, if you are in my lab make sure you remember to look at the online pre-lab and take the quiz on D2L.

Exam #1, Fall 2008

I posted a key for last year's exam if you'd like to check your answers.

www.mnstate.edu/bodwin, in the left panel click on "Gen Chem I" under the "Previous" heading.

Review...

Today in class we reviewed for Monday's exam. We went over the questions from the in-class problem set, the answers were: cobalt has a +3 charge, the alloy is 85% iron.

If you have other questions, let me know and I will answer them to the blog. Don't forget about your MasteringChemistry assignments.

Exam reminders:

- At least 1 empty seat between everyone

- No graphing calculators, no cell phone calculators, and turn off your phones

See you all Monday.

Naming organics

An email question...

what do the numbers in front of the name mean? (for instance 2 pentene or 4 nonene)

The numbers indicate the location of the functional group. Many (most?) organics exist as chains of carbon atoms so if we want to draw a correct picture of the molecule based only on the name, it's necessary to specify where on the chain to put functional groups. For alkenes, the molecule contains a double bond, so if we start with pentane:

CH3-CH2-CH2-CH2-CH3

and want to make one of those bonds a double bond (and remove 2 hydrogens) to form pentene, we could either make the first bond in the chain a double or the second, so we need to specify whether it is 1-pentene or 2-pentene. The same is true for other functional groups, 1-propanol has the -OH group on the first carbon, 2-propanol has the -OH group on the second carbon.

This comes up in your MasteringChemistry assignment, but it is not a super-important distinction for us right now. The chemical formula of 1-propanol is C3H7OH, and the chemical formula of 2-propanol is C3H7OH, so for purposes of calculating mass or moles they can be treated the same. When we start looking more closely at properties of molecules, we will start to see differences between them, but that's a few chapters away at this point.

Exam questions

And a reminder, we will have time to review on Friday so if you have questions you would like me to go over, let me know ahead of time or bring them with you. If we don't have questions, we will start the next chapter -- balancing equations......

Almost exam time...

Today in class we talked about naming organic compounds using roots and suffixes. The classes of organic compounds we covered were: alkanes, alkenes, alkynes, alcohols, amines, and acids. {Hmm, they all start with "a"...} There's a table in the book that lists others, but I think the "a" team is enough for us at this point.

We also looked at the difference between empirical formulas and molecular formulas. Molecular formulas are always multiples of empirical formulas (even if you're multiplying by "1" in some cases).

A few people have contacted me about the lab quiz on D2L. You will only have a D2L lab quiz if you are in my lab classes; I don't believe Dr. Marasinghe is using D2L lab quizzes. Labs will meet in the same rooms as last week to get started (BR263 for my 9am Thursday lab, SL118 for all others.

We also worked on a problem set in class, I've copied it below.

Chem 150 – Fall 2008 – Problem Set #1
1. You have found a bottle in your lab that is labeled “cobalt sulfite”, but the charge of the cobalt is not listed. After analysis, you find that the contents have the following composition: % Co = 32.91; % S = 26.86. What is the correct formula of this compound and what is the charge of the cobalt?
2. You have received a sample of an iron-nickel alloy (a mixture of metals) that contains exactly 1.00mol of metal atoms. This sample has a mass of 56.267g. What is the percent iron in this sample? {Hint: This is a weighted average problem, just like isotopic abundance, but it’s using 2 different elements instead of 2 different isotopes.}

OK, this post is getting kind of long. You have a new MC assignment posted, due Sunday.

Chemical formulas...

Today we talked about ionic and molecular compounds, including naming, writing formulas, and determining formulas from percent composition data. You will be expected to know all of the polyatomic ions listed in Table 3.5 of your textbook (page 95). Flashcards might not be a bad way to learn them...

There's a new MC assignment, due next Friday.

For those of you in my labs, be sure to check D2L. I will have info posted some time today or tomorrow, including a pre-lab "quiz" that will be due Wednesday.

Have a fun and safe weekend, I'll see you all on Wednesday.

The mole

Today we talked about predicting charges on ions and we used the mole to relate microscopic amounts to macroscopic masses.

There's a new MC, due Sept. 9th.